The present invention relates to doped organic semiconductor materials having an increased charge carrier density and effective charge carrier mobility, as well as to a process for preparing them by doping with a dopant, wherein, after mixing the dopant into the organic semiconductor material, hydrogen, carbon monoxide, nitrogen or hydroxy radicals are split off and at least one electron is transferred to the semiconductor material or from the semiconductor material.
Since the demonstration of organic light-emitting diodes and solar cells in 1989 (C. W. Tang et al., Appl. Phys. Lett., 1987, 913), components constructed from thin organic layers have been the subject of intensive research. Such layers have advantageous properties for the uses mentioned, properties such as, e.g., efficient electroluminescence for organic light-emitting diodes, high absorption coefficients in the visible light region for organic solar cells, inexpensive preparation of the materials and of the production of components for the simplest electronic circuits, etc. The use of organic light-emitting diodes for display applications is already of commercial importance.
The performance characteristics of (opto)electronic multilayer components are determined, among other things, by the ability of the layers to transport the charge carriers. In the case of light-emitting diodes the ohmic losses in the charge transport layers during operation are related to the conductivity, which, on the one hand, has a direct effect on the required operating voltage, and on the other hand also determines the thermal load on the component. Furthermore, there develops, as a function of the charge carrier concentration of the organic layers, a band deformation in the vicinity of a metal contact, which can facilitate the injection of charge carriers and thereby reduce the contact resistance. In the case of organic solar cells, too, similar considerations lead to the conclusion that their efficiency is also determined by the transport characteristics for charge carriers.
By doping hole transport layers with a suitable acceptor material (p-doping) or electron transport layers with a donor material (n-doping) the charge carrier density in organic solids (and hence the conductivity) can be considerably increased. Moreover, in analogy to the experience with inorganic semiconductors, applications can be expected which are based precisely on the use of p- and n-doped layers in a component and which otherwise would not be conceivable. U.S. Pat. No. 5,093,698 describes the use of doped charge carrier transport layers (p-doping of the hole transport layer by admixture of acceptor-type molecules, n-doping of the electron transport layer by admixture of donor-type molecules) in organic light-emitting diodes.
Compared with doping processes with inorganic materials, which, on the one hand, lead to problems of diffusion of the doping material used in the form of relatively small molecules or atoms, and, on the other hand, to undesired unpredictable chemical reactions between matrix and doping material, the use of organic molecules as doping material has been found advantageous. In general, organic dopants show a higher stability of the components, and diffusion plays a subordinate role, so that the defined preparation of sharp transitions from p-doped to n-doped regions is simplified. On doping with organic molecules a charge transfer exclusively takes place between matrix and doping material, but no chemical bond is formed between the latter. Furthermore, in the case of organic dopants the doping concentration to obtain a high conductivity of the doped layer is, advantageously, about three to four orders of magnitude below that of inorganic dopants.
The use of organic dopants is disadvantageous in that the desired doping materials, which are distinguished by extreme electron affinities or reduction potentials for p-doping or corresponding ionization potentials or oxidation potentials for n-doping, have a low chemical stability and are difficultly accessible.
A. G. Werner et al., Appl. Phys. Lett., 2003, 4495–7 shows the use of pyronine B as doping material for organic semiconductor materials. To this end, pyronine B chloride is sublimed, which essentially results in the formation of hydrogen chloride and a reduced, protonated form of pyronine B, namely the leuco base of pyronine B. The leuco base is used as a stable precursor substance which, through vacuum evaporation in the presence of a matrix material of low acceptor strength, is again oxidized to a cation.
A drawback of this process is due to the reaction to form the leuco form of the cationic dye. In this reaction, polymerization of pyronine B takes place, which results in an only limited sublimability of pyronine B chloride.
Furthermore, H. Yokoi et al., Chemistry Letters, 1999, 241–2, describes a photochemical doping of tetrafluorotetracyanoquinodimethane (TCNQ) by photo-induced electron transfer and C—C splitting of the radical cation, where TCNQ is mixed with dimers of the dopant, e.g., di(p-methoxyphenylamine)methyl, by grinding in a mortar. On illumination, oxidation of the dimer and electron transfer to TNCQ take place. During the oxidation, the C—C bond is split and a monomer cation and a monomer radical are formed. Through electron transfer to TCNQ, the monomer radical is oxidized to the cation.
This process is disadvantageous in so far as its use in vacuo is not possible and hence it cannot be used in combination with the usual organic semiconductor materials, such as, e.g., fullerene C60. In principle, there is a direct reciprocal relationship between the stability of the dimer and that of the cation monomer, so that a cation monomer that is all the more stable is, as uncharged dimer, only of limited stability.